Field
Aspects of embodiments of the present invention are directed to devices using spin-torque oscillators or spin-transfer oscillators.
Related Art
Spin-torque oscillators, also known as spin-transfer oscillators or spin-transfer nano-oscillators are magnetic multilayer devices that generally include two conducting magnetic layers: a reference layer (or pinned layer or polarizer layer) having a fixed magnetization, and a free layer (e.g., an isotropic free layer) having a magnetization that is free to rotate (e.g., vortex precession or skyrmion precession) in response to a current generated spin torque.
For example, FIG. 1 depicts a comparative magnetic tunneling junction (MTJ) 1. The comparative MTJ 1 typically resides on a bottom contact 2, and may include a conventional driving reference layer 10, a conventional tunneling barrier layer 20, a conventional free layer 30 (e.g., an isotropic free layer or having an anisotropy parallel to or perpendicular to the x-y plane), a second conventional tunneling barrier layer 40, conventional readout reference layer 50, and a top contact 4.
The bottom and top contacts 2 and 4 are used to drive the current from a current source in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The magnetic tunneling junction 1 may also include one or more seed layers (e.g., between the bottom contact and the conventional driving reference layer 10) and may include an antiferromagnetic (AFM) layer. The one or more seed layers may aid in the growth of subsequent layers, such as the AFM layer, having a desired crystal structure. The conventional tunneling barrier layers 20 and 40 are nonmagnetic and may be, for example, a thin insulator such as MgO.
The conventional driving reference layer 10 and the conventional free layer 30 are magnetic. The magnetization 12 of the conventional driving reference layer 10 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer. For example, as shown in FIG. 1, the magnetization 12 of the conventional driving reference layer 10 is fixed along the direction perpendicular to the plane (e.g., aligned along the z direction, which is perpendicular to the x-y plane). Although depicted as a simple (single) layer, the conventional driving reference layer 10 may include multiple layers. For example, the conventional driving reference layer 10 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such an SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic. Further, other versions of the comparative MTJ 1 might include an additional pinned layer (not shown) separated from the conventional free layer 30 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 30 has a changeable magnetization or magnetic moment 32. Although depicted as a simple layer, the conventional free layer 30 may also include multiple layers. For example, the conventional free layer 30 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru.
The conventional readout reference layer 50 has a fixed magnetization 52. The fixed magnetization 52 is in a direction such that the angle between its direction and the direction of the changeable magnetization 32 of the conventional free layer 30 varies as the changeable magnetization 32 rotates or precesses. As such, the resistance between the bottom contact 2 and the top contact 4 varies over time in accordance with the inner product of the changeable magnetization 32 of the conventional free layer 30 and the fixed magnetization 52 of the conventional readout reference layer 50.
In general, spin-torque oscillators are of interest due to their small size, easy fabrication using standard silicon processing, radiation hardness, and the frequency of oscillation can be set based on current and the strength of an externally-applied magnetic field.
However, comparative spin-torque oscillators such as that described above present a number of challenges, including requirements for the externally-applied magnetic field to be large (e.g., on the order of 1,000-10,000 Oe), and it may be difficult to modulate the frequency because doing so requires changing the externally-applied magnetic field. In addition, the output power is generally very low (e.g., on the order of 1-10 nW) and comparative oscillators have high current requirements (e.g., on the order of a few mA).
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not constitute prior art.